Conventional processes for producing expandable styrene-based polymer beads are roughly classified into a process comprising impregnating a blowing agent into styrene-based polymer beads previously prepared by suspension polymerization and a process comprising suspension polymerization of styrene-based monomers in the presence of a blowing agent to produce particulate polymers containing the blowing agent.
The first process requires preparation of styrene-based polymer particles by suspension polymerization in advance. Since the styrene-based polymer particles obtained by suspension polymerization have a continuous broad particle size distribution of from small to large particles, they should be sieved to obtain a fraction having a desired narrow particle size distribution, which is then impregnated with a blowing agent. Therefore, this process involves an increased number of production steps, i.e., the suspension polymerization step, the sieving step, and the blowing agent impregnation step.
On the other hand, the second process does not involve one of the previous suspension polymerization for obtaining styrene-based polymer particles and the impregnation of a blowing agent and thus has an advantage of a smaller number of steps required. However, since the blowing agent is impregnated into all the produced polymer particles having a broad particle size distribution, there is a great problem on handling of the blowing agent-containing particles having particle sizes unsuitable for use as expandable beads. More specifically, of the expandable particles containing the blowing agent as produced by the second process, those having a particle size in the range of from 0.5 to 2.0 mm are most generally employed as expandable beads and are put on the market as on-specification materials, while those having particle sizes out of the above range can find almost no market. The incidental production of off-specification particles has been a chief cause of increasing the production cost of the expandable styrene-based polymer beads according to the second process.
In an attempt of overcoming the disadvantages of the aforesaid first and second processes, there has been proposed a process for producing expandable styrene-based polymer beads having a narrow particle size distribution, which comprises suspending classified small particles of a styrene-based polymer containing no blowing agent in an aqueous medium, quantitatively adding a styrene-based monomer having dissolved therein a polymerization initiator to the suspension system, conducting suspension polymerization until the polymer particles grow to a desired particle size, and then impregnating a blowing agent into the particles to thereby obtain expandable styrene-based polymer beads having a narrow particle size distribution, as disclosed in British Patent No. 1,255,237, West German Patent No. 1,917,090, Canadian Pat. No. 890,463 or French Patent No. 2,005,711 each corresponding to Japanese Patent Publication No. 2987/71. This improved process can produce expandable styrene-based polymer particles having a considerably narrow particle size distribution as compared with the conventional processes. Nevertheless, there still is a disadvantage that fine powdery polymer particles are produced in a proportion less than about 10%. Further, since this process starts with styrene-based polymer particles containing no blowing agent which are prepared by suspension polymerization followed by classification, the number of steps required is so increased.
Taking into consideration the advantages of the above-described second process in terms of number of steps involved, it would promise great benefit upon the industry to develop a process for converting the blowing agent-containing small particles, particularly those having a particle size less than 0.5 mm, that are unsuitable for use as expandable beads as obtained by the second process into particles having a size permitting of a use as expandable beads, e.g., of from 0.5 to 2.0 mm.